Atlantis was originally populated by humans (or near-humans). When the city sank beneath the waves, one of the greatest scientists constructed a dome over the city to protect the people. ("The Girl in Superman's Past!")

Supergirl first visited Atlantis in 1960, when she saved the city from some ancient weapons which had accidentally been freed from a crypt and reactivated. She was then given a tour of Atlantis by Jerro. ("Supergirl's First Romance!")

Peoples of Atlantis

Different sources have shown at least three different races inhabiting Atlantis.

A life-size statue of one of the six-inch Atlanteans.

A tiny race, alienesque but humanoid in shape, and only six inches tall. They used Gold Flying Saucers to explore space. This race has apparently died out, though their fate is unknown. Also unknown is why the contemporary Atlantean races never mention them. ("The Secret of the Space Souvenirs")

A race of Merpeople. They are telepathic; descended from a Human race, but genetically manipulated to be able to survive in Atlantis prior to its destruction. Able to communicate with marine life. ("The Girl in Superman's Past!")